1. Technical Field
The field relates broadly to sonar and more particularly to transducer arrays and to sound field shaping and focusing elements for use with transducer arrays.
2. Description of the Problem
In active sonar systems arrays of acoustic transducers operate to combine the outputs from a large plurality of transducers to combine the sound waves from the individual transducers to narrow and intensify the sound field into a sound beam and, by selectively weighting or adjusting the phase of the input signal to individual transducers, to direct the beam. Typically the transducers have been deployed in a plane arranged in rows and columns, though other approaches are known, see for example WO 2006/016156. Directivity is improved and dispersion reduced with each additional row or column of transducers, however, improved directivity is largely a byproduct of increased destructive summation between the sound fields produced by individual transducers. While such arrays allow beams of high intensity to be generated, the gains obtained with each additional row or column of transducers quickly grow small in comparison to the number of transducers added at each step. While such diminishing returns are characteristic of many systems, they are pronounced in flat matrix like arrays.
Parabolic dishes have also been used to reduce the dispersion of a sound field to produce a beam. Meyer et al., in U.S. Pat. No. 5,821,470, describe a Broadband Acoustical Transmitting System based on a parabolic reflector incorporating two loudspeaker transducers. Meyer may be taken as representative of such systems in that he provided one transducer be spaced from the dish, forward along the intended axis of propagation of sound and located at the focal point of the dish. The focal point transducer was horn loaded and oriented to propagate sound backward along the radiant axis and into the dish for reflection in a collimated beam. Meyer added a second transducer for low frequency sound. The second transducer was located opposed to the horn loaded transducer and on the radiant axis of the dish, flush mounted in the dish and oriented for forward propagation of sound centered on the radiant axis. At this location the low frequency transducer derives relatively little benefit from the dish as a focusing element, though the dish still serves as a baffle.
Underwater systems, particularly sonar systems, frequently rely on piezoelectric transducers which can be readily applied both to the efficient generation of sound and for the efficient generation of electrical signals in response to impinging sound. Thus arrays of such transducers are more readily applied both for sound generation and sound reception when immersed in water than systems using transducers optimized for use in a compressible medium such as those built for atmospheric use.